Known catalyst support materials include zeolite materials. Such zeolites typically degrade at temperatures less than or equal to 700.degree. C. with collapse of pore structure and loss of surface area.
The preparation of sub-micron silicon carbide powders that are suitable for fabrication of structural ceramic bodies by pressure or pressureless sintering processes is described in many references. Wei, Morgan, Kennedy, and Johnson, Synthesis, Characterization and Fabrication of Silicon Carbide Structural Ceramics, Annual Conference Material Coal Conversion Utility (Process) 7, 187 (1982), describe a variety of sol-gel routes to synthesis of silicon carbide powders. These routes include synthesis of silicon carbide by gelation of colloidal silica with petroleum pitch and toluene and by polymerization of methyl trimethoxysilane and phenolic resin in alcohol and water. Preparation and characterization of these sols and resulting silicon carbide powders is also described in Wei, Kennedy and Harris, Ceramic Bulletin, volume 63, pages 1054-1061 (1984). After pyrolysis of the gels at from 500.degree. C. to 800.degree. C. they are reacted at 1600.degree. C. under argon in a graphite furnace to form fine silicon carbide powders which are typically oxidized at 550.degree. C. to 600.degree. C. in air to remove excess carbon and treated with hydrofluoric acid to eliminate residual silica. The resulting powders exhibited surface areas of up to 59 m.sup.2 /g due to their submicron particle size.
Japanese patent publication J-58091-027-A discloses the manufacture of silicon carbide powder by reducing at 1400.degree. C. to 1850.degree. C. a mixture of carbon and silicon dioxide powders and thereafter calcining in a non-oxidizing atmosphere. Methylsilicate (CH.sub.3 SiO.sub.1.5) is obtained by hydrolysis of methyltrichlorosilane. To this is added carbon powder, for example, carbon black, graphite, lampblack, or resin capable of resulting in carbon upon calcination.
Japanese patent publication J-58091-028-A discloses manufacture of silicon carbide powder having a fine grain size which may employ as its silica source trimethylchlorosilane which includes the group (CH.sub.3 SiO.sub.1.5).sub.n. In this case there is also added an external carbon source to the gel prior to preparation of the desired silicon carbide powder, which should be less than 0.5 micron.
Japanese patent publication No. J-58091-026-A describes preparation of high purity silicon carbide powder by mixing Si alkoxide, e.g., ethylsilicate or methylsilicate, and a carbonaceous substance, e.g., amorphous carbon, graphite, carbohydrates such as fructose, sucrose, starch or cellulose with aqueous solution of acid or alkali at 20.degree. C.-80.degree. C. to form a gel; drying the gel at 80.degree. C.-200.degree. C. for several hours, and subsequently calcining at 1300.degree. C.-1800.degree. C. under a non-oxidizing atmosphere.
Japanese patent publication No. J-57017-412 describes the preparation of finely powdered metal carbides having a grain size less than 1 micron which are prepared by reacting metal halide or alkoxide with carbohydrates such as glucose, gelactose, arabinose, saccharose, starch, cellulose, etc., and calcining the resultant at 700.degree. C.-2300.degree. C. for 1-3 hours. Examples of suitable metal alkoxides include tetramethoxysilane, tetraisopropoxysilane, dimethyl diethoxysilane, tetraethoxyZirconium, tetra-n-butoxyZirconium, dicholoro-tri-n-propoxyTantalum and dichlorotetraethoxyTungsten.
European patent publication No. 0052487 describes a method for manufacture of silicon carbide using liquid silicic acid or modified liquid silicic acid as a siliceous substance and carbon in powdered form, a precursor of carbon in a powdered form, or a precursor or carbon in the form of a solution, which are combined in the range of 0.3 to 5 parts per weight of carbon or precursor of carbon to liquid silicic or modified liquid silicic acid. Gelling is to be avoided prior to bringing the two substances into a homogenous liquid state. After combination, the precursor material may be dried or directly subjected to heat or precipitated. Thereafter, the mixture is heated in a non-oxidative atmosphere at temperatures of 1350.degree. C. to 1850.degree. C. to form silicon carbide particles of submicron diameter.
Cannon, Danforth, Haggerty and Marra, in an article entitled "Sinterable Ceramic Powders From Laser-Driven Reactions: II Powder Characteristics and Process Variables", Journal of American Ceramic Society, volume 65, No. 7, pages 330-335 (1982) describe laser-driven reactions of SiH.sub.4 and C.sub.2 H.sub.4 to produce silicon carbide powders of less than 1/10 micron particle size, having a surface area between 84 and 97 m2/g.
The preparation of ultrafine, ultrapure silicon carbide powder has been achieved using plasma-assisted chemical vapor deposition from reactant gases SiH.sub.4 and CH.sub.4. This work is described in an article entitled "RF-Plasma System For The Production of Ultrafine/Ultrapure SiC Powder" published by Basic Science Division of the American Ceramic Society, authored by Hollabaugh, Hull, Newkirk and Petrovik of Los Alamos National Laboratory.
U.S. Pat. No. 4,460,639 discloses preparation of fiber-reinforced glass matrix composites in which pyrolyzed (RSiO.sub.1.5).sub.n acts as the matrix. (RSiO.sub.1.5).sub.n gels may also be expressed as organosilsesquioxane. According to this patent, gels were typically pyrolyzed at temperatures of less than or equal to 1200.degree. C. Formation of silicon carbide is not indicated.
Andersson and Warren, in an article entitled, "Silicon Carbide Fibers and Their Potential Use in Composite Materials, Part I", Composites, volume 15, pages 16-24, indicate silicon carbide fibers have been prepared by controlled pyrolysis of polycarbosilane polymers melt-spun into fiber form. The commercial production of these fibers was begun by Nippon Carbon Company under the trade name Nicalon in 1981. The chemistry involved in the preparation of these fibers is as follows: ##STR1## Such fibers may be characterized as a partially amorphous, partially crystalline mixture of silicon carbide, silica and carbon. Pyrolysis temperatures of 1300.degree. C. or less in hydrogen or vacuum are employed. The result is the formation of a low surface area, low porosity fibrous material having attractive mechanical properties. Pyrolysis at higher temperatures is considered undesirable due to loss of mechanical properties. Of particular concern is carbothermic reduction of silica by carbon resulting in carbon monoxide evolution. It is stated that up to at least 1500.degree. C., the reaction is expected to affect mainly the fiber surface and that significant rates are not observed below about 1200.degree. C., even in the most reactive systems.